I saw some discussions regarding QoS settings not influencing latency on a
loaded link, or not influencing it enough.
The following formula may help clarify things:
avgpacketbytes*5000
msavglatencyadded= -------------------
bpsoflink
So for a 56k6 modem, this boils down to 132ms on average *added* to the
existing latency. Why is this so? There is a baseline latency, which is a
given. Then there is a queue of packets that are destined to go out before
yours.
In the absence of QoS control, that queue may be huge. Perfect QoS would
lead to a zero sized queue ahead of your packet. However, as packets can't
be subdivided or interrupted (generally), your modem will already be busy
sending one packet when yours gets queued for transmission.
On average, this packet will be halfway when your packet arrives. So, the
additional latency in msec is the amount of time it takes your link to
transmit half a typical packet.
The peak additional latency is twice that calculated above, so you could
also see 264ms additional delay.
This is an artifact of having packets which take a long time to transmit.
You can ameliorate the problem by setting a lower MTU to make sure your
packets go out quickly.
Regards.
bert
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